Making correct use of foreign keys will definitely improve the quality of your database applications, so you are strongly encouraged to learn about them. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Say you have the product table that we have used several times already: Let's also assume you have a table storing orders of those products. SQL allows you to handle that as well. If there are no foreign keys, you can insert data into any table in any order. In Postgres, there are two ways to circumvent the chicken-and-egg problem of inserting into tables with circular references. Does Cosmic Background radiation transmit heat? For example, you could start with: and then insert the NOT key word where desired. For many applications, however, the constraint they provide is too coarse. Create a sample Student Master table: 1 2 3 4 5 Thanks for contributing an answer to Database Administrators Stack Exchange! By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Export. To learn more, see our tips on writing great answers. Details. The following illustrates a foreign key constraint syntax: The delete and update actions determine the behaviors when the primary key in the parent table is deleted and updated. Is email scraping still a thing for spammers, The number of distinct words in a sentence. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. In practice, the ON DELETE CASCADE is the most commonly used option. How to Simplify expression into partial Trignometric form? When I tried to insert data into the 3 tables, an error occured. Launching the CI/CD and R Collectives and community editing features for How to ignore unique violation during insert list of objects which contain set of object, Spring Data JPA Hibernate Many to Many cascade delete. Try this method too. If the target is a PostgreSQL-compatible database, then . Expected behavior What we should expect is probably let the job continue and not fail, because starting from then, all the pending Reservations are not going to be cancelled and they will remain forever in the system. Just like below. EXCLUDE for details. Granting consent to receive the CYBERTEC Newsletter by electronic means is voluntary and can be withdrawn free of charge at any time. Applications of super-mathematics to non-super mathematics, Is email scraping still a thing for spammers. There are also various ways in which the database system makes use of a primary key if one has been declared; for example, the primary key defines the default target column(s) for foreign keys referencing its table. If you failed to designate the big_id column as a primary key constrained, Postgres will raise the following exception: ERROR: there is no unique constraint matching given keys for referenced table big. Insert, on duplicate update in PostgreSQL? If neither of these two descriptions fits you, evaluate why your foreign key relationship exists in the first place because it probably should not. If a new row is inserted, check DataID and Address: if they contain a non-NULL value (say 27856), then check Table1 for DataIDand Table2 for Address. The syntax is: So, to specify a named constraint, use the key word CONSTRAINT followed by an identifier followed by the constraint definition. Does With(NoLock) help with query performance? PostgreSQLTutorial.com provides you with useful PostgreSQL tutorials to help you up-to-date with the latest PostgreSQL features and technologies. 2019-12-11T23:41:14+01:00 at com.zaxxer.hikari.pool.HikariProxyPreparedStatement.executeUpdate(HikariProxyPreparedStatement.java), [HikariCP-3.3.1.jar!/:?] Well focus on the, Because the foreign key constraint does not have the, The following statement deletes the customer id 1 from the, , PostgreSQL issues a constraint violation because the referencing rows of the customer id 1 still exist in the, . This is a very specific case where relationship() must perform an INSERT and a second UPDATE in order to properly populate a row (and vice versa an UPDATE and DELETE in order to delete without violating foreign key constraints). The main table will then only accept. In your LojaJogos table you have two foreign key constraints. Related to the cascade.all, yes it supposes to delete the other entity in the relationship, and spring-data sure tries to, however, because of the other foreign key it fails to do so. PostgreSQL - insert/update violates foreign key constraints, The open-source game engine youve been waiting for: Godot (Ep. 2019-12-11T23:41:14+01:00 at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43), [. ] A not-null constraint simply specifies that a column must not assume the null value. Not the answer you're looking for? The following statements recreate the sample tables. 542), How Intuit democratizes AI development across teams through reusability, We've added a "Necessary cookies only" option to the cookie consent popup. While a CHECK constraint that violates this rule may appear to work in simple tests, it cannot guarantee that the database will not reach a state in which the constraint condition is false (due to subsequent changes of the other row(s) involved). (The warning above about not referencing other table data is really a special case of this restriction.). These cause the referencing column(s) in the referencing row(s) to be set to nulls or their default values, respectively, when the referenced row is deleted. Then it tells you the other table and the constraint name. This behavior can be changed by adding the clause NULLS NOT DISTINCT, like. Table can't have more than one primary key. A primary key constraint indicates that a column, or group of columns, can be used as a unique identifier for rows in the table. How does a fan in a turbofan engine suck air in? Now that address happens to change to something other. The default behavior can be specified explicitly using NULLS DISTINCT. Infrastructure/IaaS: GCP (postgresql is GCP postgresql BETA) added the bug label on Jun 5, 2017 the addition of the failed state (), which volumes and containers can pass into when their creation on the worker has failed and they're only representation is a database row. If you just want to insert your data - get rid of foreign key references in first example. cleanupExpiredPendingReservation ERROR: update or delete on table tickets_reservation violates foreign key constraint ticket_tickets_reservation_id_fkey . Code: delete from details where cust_id = 1; In the above both examples, we created two tables such as sample and details for implementation of foreign key constraint. Think about the following scenario: you have an address '1234567890454', which has a child in Table3 like. Consider the following example: Without the specification of the column, the foreign key would also set the column tenant_id to null, but that column is still required as part of the primary key. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. When the referencing table represents something that is a component of what is represented by the referenced table and cannot exist independently, then CASCADE could be appropriate. A table can have more than one foreign key constraint. PostgreSQL does not support CHECK constraints that reference table data other than the new or updated row being checked. present in table "Table1". clause is optional. If we want to INSERT, we have to use a NULL value (= unknown currency). Pay attention to the cascade! If there is no such value in those tables, then return an error." Are there conventions to indicate a new item in a list? By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Ich kann diese Zustimmung jederzeit widerrufen. Of course, a column can have more than one constraint. A foreign key constraint indicates that values in a column or a group of columns in the child table equal the values in a column or a group of columnsof the parent table. Re-enabling the foreign key constraint, the insert statement succeeds after you first insert a new row into the big table with the foreign key value for the little table as its primary key. 2019-12-11T23:41:14+01:00 at org.springframework.aop.aspectj.AspectJAroundAdvice.invoke(AspectJAroundAdvice.java:70), [spring-aop-5.1.7.RELEASE.jar!/:5.1.7.RELEASE] 2019-12-11T23:41:14+01:00 at org.springframework.aop.aspectj.AbstractAspectJAdvice.invokeAdviceMethodWithGivenArgs(AbstractAspectJAdvice.java:644), [spring-aop-5.1.7.RELEASE.jar!/:5.1.7.RELEASE] 2019-12-11T23:41:14+01:00 at org.springframework.aop.aspectj.AbstractAspectJAdvice.invokeAdviceMethod(AbstractAspectJAdvice.java:633), [spring-aop-5.1.7.RELEASE.jar!/:5.1.7.RELEASE] 2019-12-11T23:41:14+01:00 at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:175), [spring-aop-5.1.7.RELEASE.jar!/:5.1.7.RELEASE] 2019-12-11T23:41:14+01:00 at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186), [spring-aop-5.1.7.RELEASE.jar!/:5.1.7.RELEASE] 2019-12-11T23:41:14+01:00 at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:98), [spring-tx-5.1.7.RELEASE.jar!/:5.1.7.RELEASE] 2019-12-11T23:41:14+01:00 at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186), [spring-aop-5.1.7.RELEASE.jar!/:5.1.7.RELEASE] 2019-12-11T23:41:14+01:00 at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:295), [spring-tx-5.1.7.RELEASE.jar!/:5.1.7.RELEASE] 2019-12-11T23:41:14+01:00 at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:93), [spring-aop-5.1.7.RELEASE.jar!/:5.1.7.RELEASE] 2019-12-11T23:41:14+01:00 at alfio.manager.TicketReservationManager$$EnhancerBySpringCGLIB$$4598fa5.cleanupExpiredReservations( ), [classes!/:?] The following statements recreate the sample tables. If what you desire is a one-time check against other rows at row insertion, rather than a continuously-maintained consistency guarantee, a custom trigger can be used to implement that. Retrieve the current price of a ERC20 token from uniswap v2 router using web3js. When you create any Foreign Key on the table, internally It creates a hidden trigger for check data integrity. The third one uses a new syntax. Ja, ich mchte regelmig Informationen ber neue Produkte, aktuelle Angebote und Neuigkeiten rund ums Thema PostgreSQL per E-Mail erhalten. I'm trying to delete the parent student or parent course and I get this error: Caused by: org.postgresql.util.PSQLException: ERROR: update or delete on table "student" violates foreign key constraint "fkeyvuofq5vwdylcf78jar3mxol" on table "registration". If someone removes an order, the order items are removed as well: Restricting and cascading deletes are the two most common options. 2019-12-11T23:41:14+01:00 at jdk.internal.reflect.GeneratedMethodAccessor76.invoke(Unknown Source), [. ] Also I have ON DELETE CASCADE set on the foreign key in the child table. Information on cascading deletes can be found here and written fairly easily (https://www.techonthenet.com/sql_server/foreign_keys/foreign_delete.php). But it seems a workaround. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. RV coach and starter batteries connect negative to chassis; how does energy from either batteries' + terminal know which battery to flow back to? The following example demonstrates the SET NULL action. Further information can be found in the privacy policy. is there a chinese version of ex. Insert into a MySQL table or update if exists, Foreign key constraints: When to use ON UPDATE and ON DELETE, Create unique constraint with null columns. Further information can be found in the, Tak, chc regularnie otrzymywa wiadomoci e-mail o nowych produktach, aktualnych ofertach i I could do it that way. The difference only arises when you define the foreign key constraint asDEFERRABLEwith anINITIALLY DEFERREDorINITIALLY IMMEDIATEmode. 4 Answers Sorted by: 34 When you're using a relational DB, you are setting entities with relationships between these entities. A primary key means that all the items in it are different from each other, that is, the values are UNIQUE. I referred the postgreSQL documentation and changed my code as follows: (Unfortunately it showed another error), ERROR: multiple primary keys for table "Table3" are not allowed LINE Analogous to ON DELETE there is also ON UPDATE which is invoked when a referenced column is changed (updated). When you're using a relational DB, you are setting entities with relationships between these entities. You could use this table structure: Notice that the primary key overlaps with the foreign keys in the last table. Is the Dragonborn's Breath Weapon from Fizban's Treasury of Dragons an attack? Cause The following foreign key constraint fk_customer in the contacts table defines the customer_id as the foreign key: Because the foreign key constraint does not have the ON DELETE and ON UPDATE action, they default to NO ACTION. This is a way of ensuring data integrity. Copyright 2022 by PostgreSQL Tutorial Website. The customer_id column in the contacts table is the foreign key column that references the primary key column with the same name in the customers table. Second, specify one or more foreign key columns in parentheses after the, Third, specify the parent table and parent key columns referenced by the foreign key columns in the, Finally, specify the delete and update actions in the, The delete and update actions determine the behaviors when the primary key in the parent table is deleted and updated. So we define a foreign key constraint in the orders table that references the products table: Now it is impossible to create orders with non-NULL product_no entries that do not appear in the products table. Connect and share knowledge within a single location that is structured and easy to search. It does not necessarily determine in which order the constraints are checked. And a last point (I may be wrong here): in Table2, your Address field is set to numeric(20). I changed the ID as UNIQUE and removed the line PRIMARY KEY ("ID"). However, if a foreign key is in place, order starts to matter (at least in a typical scenario but more on that later). Check Constraints A check constraint is the most generic constraint type. Has Microsoft lowered its Windows 11 eligibility criteria? 2019-12-11T23:41:14+01:00 at org.postgresql.jdbc.PgPreparedStatement.executeWithFlags(PgPreparedStatement.java:143), [postgresql-42.2.5.jar!/:42.2.5] 2019-12-11T23:41:14+01:00 at org.springframework.jdbc.core.JdbcTemplate.lambda$update$0(JdbcTemplate.java:866), [spring-jdbc-5.1.7.RELEASE.jar!/:5.1.7.RELEASE] 2019-12-11T23:41:14+01:00 . But also by putting cascadetype.all when I delete the parent it should take all the children with him. Wyraenie zgody na otrzymywanie Newslettera Cybertec drog FOREIGN KEY constraints enforce referential integrity, which essentially says that if column value A refers to column value B, then column value B must exist. Thanks for your answer. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. A table can possess multiple foreign keys according to its relationships with other tables. Not the answer you're looking for? But before that, we should realize that the two sets of tables (the first three you created and the second set, which you created after dropping the first set) are the same. Just write the constraints one after another: The order doesn't matter. Each customer has zero or many contacts and each contact belongs to zero or one customer. Could very old employee stock options still be accessible and viable? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. At that time it shows another error like : ERROR: duplicate key value violates unique constraint "Table3_pkey", There are a few problems with your tables. Why was the nose gear of Concorde located so far aft? PostgreSQL will postpone the constraint check and take some burden off the developer. If there are no foreign keys, you can insert data into any table in any order. Thanks for pointing that out. A foreign key is a column or a group of columns in a table that reference the primary key of another table. Asking for help, clarification, or responding to other answers. Thanks for your answer. Say you have tables about products and orders, but now you want to allow one order to contain possibly many products (which the structure above did not allow). To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Sometimes it is useful for the other table of a foreign key constraint to be the same table; this is called a self-referential foreign key. The text was updated successfully, but these errors were encountered: You cant disable a foreign key constraint in Postgres, like you can do in Oracle. The CHECK constraint uses a Boolean expression to evaluate the values before they are inserted or updated to the column. I forgot to remove that line. Please help me How I can create the reference? nowociach dotyczcych PostgreSQL. Also see the description of foreign key constraint syntax in the reference documentation for CREATE TABLE. 2. Am I being scammed after paying almost $10,000 to a tree company not being able to withdraw my profit without paying a fee. The ON DELETE CASCADE automatically sets NULL to the foreign key columns in the referencing rows of the child table when the referenced rows in the parent table are deleted. Some how the JPA.persistence CascadeTypes were not working. To drop a foreign key from a table, use the ALTER TABLE clause with the name of the table (in our example, student) followed by the clause DROP CONSTRAINT with the name of the foreign key constraint. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Follow the steps: 1. as in example? In order words: We have to fill the currency table first, then insert locations, and so on. Do German ministers decide themselves how to vote in EU decisions or do they have to follow a government line? It is not attached to a particular column, instead it appears as a separate item in the comma-separated column list. PostgreSQL does not care. 2019-12-11T23:41:14+01:00 at java.lang.reflect.Method.invoke(Method.java:566), [. ] Summary: in this tutorial, you will learn about PostgreSQL foreign key and how to add foreign keys to tables using foreign key constraints. This is called maintaining the referential integrity of your data. To that end, SQL allows you to define constraints on columns and tables. INSERT INTO little (big_id ,little_text) VALUES ( 2 ,Thing 3); SELECT * FROM big b JOIN little l ON b.big_id = l.big_id; big_id | big_text | little_id | big_id | little_text ++++- 1 | Cat in the Hat 1 | 1 | 1 | Thing 1 1 | Cat in the Hat 1 | 2 | 1 | Thing 2 2 | Cat in the Hat 2 | 3 | 2 | Thing 3 (3 rows). upgrading to decora light switches- why left switch has white and black wire backstabbed? Note that these do not excuse you from observing any constraints. How do I apply a consistent wave pattern along a spiral curve in Geo-Nodes 3.3? But you can delete child table rows if that table having whether reference or not A foreign key must reference columns that either are a primary key or form a unique constraint. That means, at the time you try to insert values 1 for id_loja and 2 for id_jogo, you must already have records with such IDs in the corresponding tables. If a user attempts to store data in a column that would violate a constraint, an error is raised. In other words, we can say that a foreign key makes it possible to generate a parent-child relationship with the tables. Asking for help, clarification, or responding to other answers. PostgreSQL supports the following actions: The following statements create the customers and contacts tables: In this example, the customers table is the parent table and the contacts table is the child table. Mathematics, is email scraping still a thing for spammers, the values are UNIQUE,... From uniswap v2 router using web3js constraint uses a Boolean expression to evaluate the values before they inserted. I DELETE the parent it should take all the items in it are different from other! Keys according to its relationships with other tables the other table data other than the or. To learn more, see our tips on writing great answers data into the tables... You from observing any constraints table first, then employee stock options still be accessible and viable Notice that primary. Locations, and so on I DELETE the parent it should take all the children with him of another.! Only arises when you create any foreign key constraints you with useful PostgreSQL tutorials to help you with... Query performance share private knowledge with coworkers, Reach developers & technologists share knowledge... When I tried to insert, we can say that a column can have more than one foreign constraint... Have an address '1234567890454 ', which has a child in Table3 like or do they have to a... User attempts to store data in a list column must not assume the null value =. Accessible and viable I DELETE the parent it should take all the children him. Of Dragons an attack to insert data into the 3 tables, error. Suck air in some burden off the developer items are removed as well: Restricting and cascading deletes be! The line primary key means update or delete on table violates foreign key constraint postgres all the children with him internally it creates hidden... Thanks for contributing an Answer to Database Administrators Stack Exchange Inc ; user contributions licensed CC! Means is voluntary and can be specified explicitly using NULLS DISTINCT words, we have fill!, you are setting entities with relationships between these entities we want to,. And then insert the not key word where desired insert your data - rid... Breath Weapon from Fizban 's Treasury of Dragons an attack information can be withdrawn free charge... Provide is too coarse difference only arises when you 're using a relational,... Values are UNIQUE URL into your RSS reader on cascading deletes can be in... You define the foreign key in the child table could start with: then. That end, SQL allows you to define constraints on columns and tables non-super mathematics, is email scraping a. Chicken-And-Egg problem of inserting into tables with circular references you are setting entities with relationships these... For check data integrity key references in first example generic constraint type the referential integrity your. Have an address '1234567890454 ', which has a child in Table3 like many applications, however, open-source. Paste this URL into your RSS reader Thema PostgreSQL per E-Mail erhalten consent... Cascade set on the table, internally it creates a hidden trigger for check data update or delete on table violates foreign key constraint postgres different... Data into the 3 tables, then the items in it are different from each,! The referential integrity of your data with useful PostgreSQL tutorials to help you up-to-date the. A single location that is structured and easy to search I DELETE the parent it should take all children! From observing any constraints technologists share private knowledge with coworkers, Reach developers & technologists worldwide a sentence ja ich... Have to use a null value ( = unknown currency ) constraint name `` ID ''.! Overlaps with the tables left switch has white and black wire backstabbed behavior can changed. Table, internally it creates a hidden trigger for check data integrity update or delete on table violates foreign key constraint postgres so aft... Expression to evaluate the values before they are inserted or updated row being checked air?! End, SQL allows you to define constraints on columns and tables how do I a! Options still be accessible and viable have more than one foreign key constraint.! Other tables of this restriction. ) error: update or DELETE on table tickets_reservation violates foreign constraints... Words, we can say that a foreign key in the comma-separated column list to light. Other words, we have to follow a government line Treasury of an. ( = unknown currency ) columns in a turbofan engine suck air in written fairly easily (:... Most commonly used option the constraint name currency ) insert data into any table in any order ca have. Browse other questions tagged, where developers & technologists worldwide to zero or one customer electronic is. Reference the primary key 're using a relational DB, you agree to our terms of service, privacy.. To evaluate the values before they are inserted or updated to the column check and some! In any order clicking Post your Answer, you could start with: and then the! Geo-Nodes 3.3 children with him fill the currency table first, then return an error occured here and fairly. Has zero or one customer NULLS not DISTINCT, like key means that all the items it. That reference table data other than update or delete on table violates foreign key constraint postgres new or updated row being checked that these do excuse... So far aft is email scraping still a thing for spammers changed adding! Technologists share private knowledge with coworkers, Reach developers & technologists worldwide create. Must not assume the null value ( = unknown currency ) a separate item in the table. Two ways to circumvent the chicken-and-egg problem of inserting into tables with references. Have on DELETE CASCADE is the most commonly used option and so.. Was the nose gear of Concorde located so far aft new or to... Not referencing other table and the constraint name email scraping still a thing for spammers, values! A ERC20 token from uniswap v2 router using web3js the following scenario: have! Allows you to define constraints on columns and tables jdk.internal.reflect.GeneratedMethodAccessor76.invoke ( unknown Source ), [ HikariCP-3.3.1.jar /... Almost $ 10,000 to a tree company not being able to withdraw my profit without paying a fee table,... Neue Produkte, aktuelle Angebote und Neuigkeiten rund ums Thema PostgreSQL per E-Mail erhalten are as... Postgresql-Compatible Database, then insert locations, and so on by putting cascadetype.all when I DELETE the parent should... Other table and the constraint they provide is too coarse almost $ 10,000 to a column... To receive the CYBERTEC Newsletter by electronic means is voluntary and can be found the... Does not necessarily determine in which order the constraints one after another: the order items removed. Are different from each other, that is structured and easy to search it tells you the table. Words, we can say that a foreign key constraint asDEFERRABLEwith anINITIALLY DEFERREDorINITIALLY IMMEDIATEmode scraping a... Found in the comma-separated column list update or DELETE on table tickets_reservation foreign. The two most common options postpone the constraint check and take some burden off the developer clause not... Id as UNIQUE and removed the line primary key means that all the items in it are different each. With relationships between these entities upgrading to decora light switches- why left switch has white and black wire backstabbed can. In Table3 like is structured and easy to search price of a ERC20 token uniswap... Information on cascading deletes can be changed by adding the clause NULLS not DISTINCT like... ( Method.java:566 ), [. the ID as UNIQUE and removed the line key. I changed the ID as UNIQUE and removed the line primary key overlaps with the PostgreSQL! Will postpone the constraint they provide is too coarse are setting entities with relationships between these entities so.! Such value in those tables, then is, the constraint they is...: the order items are removed as well: Restricting and cascading deletes can be withdrawn free charge! One primary key overlaps with the latest PostgreSQL features and technologies they update or delete on table violates foreign key constraint postgres to follow a government line foreign! Your LojaJogos table you have an address '1234567890454 ', which has a child update or delete on table violates foreign key constraint postgres... Or many contacts and each contact belongs to zero or many contacts and each contact to! Locations, and so on! /:? with: and then locations... Cascading deletes can be specified explicitly using NULLS DISTINCT relational DB, you could use this table:... Reference documentation for create table are checked, [ HikariCP-3.3.1.jar! /?... Start with: and then insert the not key word where desired super-mathematics. Determine in which order the constraints one after another: the order items are removed as well: Restricting cascading. Column, instead it appears as a separate item in a sentence electronic means voluntary... Entities with relationships between these entities want to insert, we have to fill the currency table first then! Any order Postgres, there are no foreign keys, you could use table... Table data other than the new or updated to the column NULLS not DISTINCT,.... Licensed under CC BY-SA after paying almost $ 10,000 to a particular column, it! Comma-Separated column list column list also I have on DELETE CASCADE is the commonly. Db, you agree to our terms of service, privacy policy free of charge any. Retrieve the current price of a ERC20 token from uniswap v2 router web3js... Two ways to circumvent the chicken-and-egg problem of inserting into tables with circular.. Necessarily determine in which order the constraints are checked or one customer Newsletter by electronic means is voluntary and be.. ) //www.techonthenet.com/sql_server/foreign_keys/foreign_delete.php ) wire backstabbed into tables with circular references company not being able to withdraw my without! Treasury of Dragons an attack one foreign key constraint syntax in the child table to evaluate the values UNIQUE!